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Showing content from https://github.com/qutip/qutip/issues/2491 below:

step does not work for SMESolver · Issue #2491 · qutip/qutip · GitHub

I wanted to use the new step method for the stochastic solver class SMESolver, but it always breaks down. I also tried it for SSESolver, but the same error message occurs. I use qutip 5.0.3. I took the example from the documentation as a minimal working example. I also tried step for the MESolver class and there it works. I think it should also not only output the state but also the measurement output if possible.

As a side note, I also wanted to know if there is a particular reason why the argument for the initial state in the start method is called state0 for MESolver, and state for SMESolver.

import numpy as np
from qutip import destroy, coherent, SMESolver, expect

# parameters
DIM = 20               # Hilbert space dimension
DELTA = 5 * 2 * np.pi  # cavity detuning
KAPPA = 2              # cavity decay rate
INTENSITY = 4          # intensity of initial state
NUMBER_OF_TRAJECTORIES = 500

# operators
a = destroy(DIM)
x = a + a.dag()
H = DELTA * a.dag() * a

rho_0 = coherent(DIM, np.sqrt(INTENSITY))
times = np.arange(0, 1, 0.0025)

stoc_solver = SMESolver(
    H,
    c_ops=[],
    sc_ops=[np.sqrt(KAPPA) * a],
    heterodyne=False,
    options={"dt": 0.00125, "store_measurement": True,}
)
data = [4.]
stoc_solver.start(state=rho_0, t0=times[0])
for t in times[1:]:
    psi_t = stoc_solver.step(t)
    data.append(expect(x, psi_t))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[4], line 29
     27 stoc_solver.start(state=rho_0, t0=times[0])
     28 for t in times[1:]:
---> 29     psi_t = stoc_solver.step(t)
     30     data.append(expect(x, psi_t))

File /user/anaconda/envs/myenv/lib/python3.9/site-packages/qutip/solver/multitraj.py:129, in MultiTrajSolver.step(self, t, args, copy)
    127     raise RuntimeError("The `start` method must called first.")
    128 self._argument(args)
--> 129 _, state = self._integrator.integrate(t, copy=False)
    130 return self._restore_state(state, copy=copy)

ValueError: too many values to unpack (expected 2)

It should have returned the state at time step t instead of failing. It would also be nice if the method would return the output of the measurement current as well.

QuTiP Version:      5.0.3
Numpy Version:      1.26.4
Scipy Version:      1.13.1
Cython Version:     None
Matplotlib Version: 3.6.0
Python Version:     3.9.19
Number of CPUs:     16
BLAS Info:          Generic
INTEL MKL Ext:      False
Platform Info:      Linux (x86_64)

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